Tuesday, December 22, 2015

A new titinopathy

De Cid R, Ben Yaou R, Roudaut C, Charton K, Baulande S, Leturcq F, Romero NB,
Malfatti E, Beuvin M, Vihola A, Criqui A, Nelson I, Nectoux J, Ben Aim L,
Caloustian C, Olaso R, Udd B, Bonne G, Eymard B, Richard I. A new titinopathy:
Childhood-juvenile onset Emery-Dreifuss-like phenotype without cardiomyopathy.
Neurology. 2015 Dec 15;85(24):2126-35.



To identify the genetic defects present in 3 families with muscular dystrophy, contractures, and calpain 3 deficiency.


We performed targeted exome sequencing on one patient presenting a deficiency in calpain 3 on Western blot but for which mutations in the gene had been excluded. The identification of a homozygous truncating mutation in the M-line part of titin prompted us to sequence this region in 2 additional patients presenting similar clinical and biochemical characteristics.


The 3 patients shared similar features: coexistence of limb-girdle weakness and early-onset diffuse joint contractures without cardiomyopathy. The biopsies showed rimmed vacuoles, a dystrophic pattern, and secondary reduction in calpain 3. We identified a novel homozygous mutation in the exon Mex3 of the TTN gene in the first patient. At protein level, this mutation introduces a stop codon at the level of Mex3. Interestingly, we identified truncating mutations in both alleles in the same region of the TTN gene in patients from 2 additional families. Molecular protein analyses confirm loss of the C-ter part of titin.


Our study broadens the phenotype of titinopathies with the report of a new clinical entity with prominent contractures and no cardiac abnormality and where the recessive mutations lead to truncation of the M-line titin and secondary calpain 3 deficiency.

1 comment:

  1. Charton K, Sarparanta J, Vihola A, Milic A, Jonson PH, Suel L, Luque H, Boumela I, Richard I, Udd B. CAPN3-mediated processing of C-terminal titin replaced by pathological cleavage in titinopathy. Hum Mol Genet. 2015 Jul 1;24(13):3718-31.


    Mutations in the extreme C-terminus of titin (TTN), situated in the sarcomeric M-band, cause tibial muscular dystrophy (TMD) and limb-girdle muscular dystrophy 2J (LGMD2J). The mutations ultimately cause a loss of C-terminal titin, including a binding site for the protease calpain 3 (CAPN3), and lead to a secondary CAPN3 deficiency in LGMD2J muscle. CAPN3 has been previously shown to bind C-terminal titin and to use it as a substrate in vitro. Interestingly, mutations in CAPN3 underlie limb-girdle muscular dystrophy 2A (LGMD2A). Here, we aimed to clarify the relationship of CAPN3 and M-band titin in normal and pathological muscle. In vitro analyses identified several CAPN3 cleavage sites in C-terminal titin that were defined by protein sequencing. Furthermore, cleavage products were detected in normal muscle extracts by western blotting and in situ by immunofluorescence microscopy. The TMD/LGMD2J mutation FINmaj proved to alter this processing in vitro, while binding of CAPN3 to mutant titin was preserved. Unexpectedly, the pathological loss of M-band titin due to TMD/LGMD2J mutations was found to be independent of CAPN3, whereas the involvement of ubiquitous calpains is likely. We conclude that proteolytic processing of C-terminal titin by CAPN3 may have an important role in normal muscle, and that this process is disrupted in LGMD2A and in TMD/LGMD2J due to CAPN3 deficiency and to the loss of C-terminal titin, respectively.